A true time delay (TTD) phase shifter is a component used in microwave and millimeter wave radar and communications systems to control the time delay imposed upon a signal along a particular signal path within a system. The most common use of TTD components is within phased array radars, where it is possible that thousands of TTD components may be necessary and would be connected to each antenna element within a large array of such elements. In such an example the TTD components would facilitate electronic steering of the transmit and/or receive direction of the antenna array. The most common implementation of TTD components using current technology is in the form of a monolithic microwave integrated circuit (MMIC), in which transistors are used to realize switches, and these switches are used to select among different sections of transmission lines of varying length, thus enabling a tuning of the time delay. In the past 3-4 years new implementations of TDD components have been developed based upon the use of radio frequency micro electro mechanical systems (RF MEMS).
Distributed micro electromechanical (MEM) transmission lines (DMTLs) are a proven solution for very high performance, low loss true time delay phase shifters. The DMTL, as known in the art, usually consists of a uniform length of high impedance coplanar waveguide (CPW) that is loaded by periodic placement of discrete MEM capacitors. The MEM devices are typically designed such that the reflection coefficient for the input, S11, for a DMTL section is less that −10 dB for the two phase states, i.e. MEM capacitors in the up- and down-state positions. The increase in the distributed capacitance in the down-state provides a differential phase shift (Δφ) with respect to the phase in the upstate.
A limitation of the capacitively-loaded DMTL known in the prior art is that the amount of phase shift is proportional to the difference in the loaded and unloaded impedances, thus restricting the achievable Δφ per unit length in light of impedance matching considerations.
Today, a large phased array radar system can cost millions of dollars. This cost can be lowered by orders of magnitude through the use of MEMS technologies. Still, there is a physical limitation to the performance achievable with RF MEMS TTD devices that operate only on the change of the capacitive loading of a transmission line. As the capacitance changes, a property of the transmission line known as the characteristic impedance (Zo) changes along with the desired change in the propagation constant. As Zo changes, there is a mismatch that arises between the TTD device and the system in which it is integrated, causing power to be reflected from the TTD device input. This mismatch is often described in terms of a parameter known as return loss (RL). A generally accepted upper limit for RL is 10 dB. The physical limitation of the capacitive only TTD device is that the amount of time delay per unit length of transmission line that can be achieved is restricted by the need to keep RL>10 dB. As one attempts to achieve greater time delay, larger changes in Zo are inherently produced, thereby decreasing the RL.
What is needed in the art is a device that improves upon the capacitance-only TTD device architecture currently known in the art. Accordingly, a device that produces true time delay phase shifting in which large amounts of time delay can be achieved without significant variation in the effective characteristic impedance of the transmission line, and thus also the input/output return loss of the component, would solve the problem of the devices currently known in the art for use in the microwave and mm-wave industry.